Jamestown, R.I. — From July through September, scuba divers and snorkelers converge in the parking lot at the Fort Wetherill State Park to explore the underwater world. But some are here for a rather unexpected reason: the tropical fish.

For years, scientists and avid divers have come to this rocky cove in Jamestown, R.I., to try to catch a glimpse of spotfin butterflyfish, snowy groupers, and other tropical varieties carried north from Florida and the Caribbean by the Gulf Stream.

“It’s not a new phenomenon,” says David Beutel, a Sea Grant fisheries extension specialist at the University of Rhode Island, Kingston (URI), but people who are used to seeing only native fish like cunner and striped bass may be surprised. “I don’t think the average person would know,” Mr. Beutel says.

During the summer, storms create warm-water eddies that split off from the Gulf Stream, carrying millions of eggs or thumbnail-size young tropical fish on a 1,500 to 2,000-mile journey up the Atlantic coast as far as Buzzards Bay, Mass.

Professor Collie says there’s been an increase over time, in tropical fish in Rhode Island waters since the university began trawling for fish in two locations in Narragansett Bay in 1959. One reason is warmer water, he says: The average temperature of Narragansett Bay has risen four degrees F. in the past 50 years. That encourages warmer-water species like butterfish and scup while also discouraging cold-water varieties such as winter flounder and silver hake. Changing fishing practices and a shift in the ecosystem are other possible factors for the decline of cold-water fish in the Bay and the rise of warmer-water ones, Collie adds.

The array of tropical fish arriving along the North Atlantic coast has grown in the past few years. Some tropical fish have been found as far north as Nova Scotia and Maine, says Todd Gardner, a biologist at the Atlantis Marine World Aquarium on Long Island, N.Y.

A particularly remarkable specimen – a red lionfish, native to the Indo-Pacific – apparently drifted up to Jamestown, R.I., in 2006. It’s still on display at the Seacoast Science Center in Rye, N.H. It’s not rare to see lionfish in the North Atlantic any more. Over the past 16 years, an influx of these venomous fish has been seen along the Atlantic coast around North Carolina and Long Island, N.Y., and as far north as Massachusetts.

Mr. Gardner says he caught the first lionfish spotted off Long Island in 2001. And in 2006, the southern bays along Long Island’s coast were filled with baby lionfish that arrived via the Gulf Stream. This year, though, he hasn’t found any lionfish. Because the Gulf Stream current is located 200 miles off Long Island, the fish that arrive along the North Atlantic coast vary from year to year. Gardner says it all depends on the contents of the small pockets of warm water from the Gulf Stream that are blown into shore every summer. “The mix of fish that we get here is almost arbitrary,” he says.

No one is sure how the lionfish invasion began, but Gardner notes that researchers began to notice an increase in the species after Hurricane Andrew in 1992. “Certainly lionfish were introduced into the Atlantic by someone or some group of people,” he says. They could not have swum or been carried by the current from halfway around the world. “Some people say it was an aquarium that got washed into an ocean,” he says.

James Morris, an ecologist at the National Oceanic and Atmosphere Administration, notes that the “lionfish are fairly low in abundance” in northern waters and don’t pose any threat as fall nears.

While warmer-water species are increasingly found farther and farther north, tropical fish haven’t been able to establish themselves in colder climes past October and November, when the ocean temperature drops below 60 degrees F. That’s why the summer months are treasured by divers at Fort Wetherill.

The stray tropical fish lured crowds there on a recent Saturday afternoon. In one corner of the parking lot, Scott Tucker and Chris Pimley of Connecticut strapped on scuba gear. They met at this rocky cove four years ago, drawn by their love of nature and documentary filmmaking. Today, they are preparing to film tropical fish for a public-access television show. Across the way, divers Diane Malczewski of West Greenwich, R.I., and Charlie Jennings of Fall River, Mass., sit on the back of a pickup truck and show off a jar holding a small, captured trunkfish they planned to take home.

Collecting tropical fish in Rhode Island is legal, and no permits are required, but regulations vary by state. These misplaced fish will surely perish as the cold sets in, so the New England Aquarium Dive Club (NEADC) and others try to catch the tropical fish before winter weather does. Harvesting tropical specimens this way makes ecological sense, since the fish have already been removed from their natural habitat.

Alicia Lenci, a member of the club, suits up with snorkel and fins. Floating close to shore, she scoops up a small trunkfish in a one-liter plastic bottle. For the past two years, NEADC has hosted a tropical fish hunt to collect fish and spread the word about the fishes’ annual arrival. Rhode Island even has a Tropical Fish Society, with 65 members. The club tries to rehabilitate and breed the tropical fish they catch.

Ms. Lenci, a volunteer at the rescue and rehab department at the New England Aquarium, and Michael Schruben of Everett, Mass., have been capturing tropical fish for 10 years and donating them to aquariums around New England. They keep a salt water tank full of cowfish, trunkfish, and spotfin butterflyfish and raise them until they are healthy enough to donate.

“I kind of think of it as a saving effort ... but I can’t save them all. I only have so much room in my apartment,” Mr. Schruben says,

Brian Nelson, senior aquarist at the New England Aquarium, helps transport the tropical fish from homes to tanks at the aquarium in Boston. He estimates that 15 percent of the fish in the aquarium’s four-story ocean tank came from Newport or Long Island. Visitors there and at the Atlantis Marine World Aquarium on Long Island probably don’t know that some of the exotic species were caught in their own backyard.